Decomposition of organic substrates at eroding vs. depositional landform positions

Abstract

Proper understanding of how rate of OM decomposition varies across a given watershed is important to determine the potential of soil erosion to induce terrestrial carbon (C) sequestration. However, as of yet, our understanding of the spatial variability of rate of organic matter (OM) decomposition (k) across a watershed is incomplete, at best. The objective of this study is to determine how rates of organic substrate decomposition vary on the surface and in soil profiles of eroding vs. depositional landform positions. To determine rate of organic substrate decomposition in eroding vs. depositional landform positions, a field litterbag decomposition study was conducted in Tennessee Valley, Northern California using in situ foliage (from grasses and a shrub) and two standard substrates (filter paper and birch tongue depressors, that served as proxies for OM that is relatively easier vs. harder to breakdown during microbial decomposition). We conducted the experiment at 3–4 depths at each landform position. The effect of erosional transport (surface to surface transfer of topsoil and associated SOM from eroding to depositional landform positions) and burial (after deposition of eroded SOM by successive erosional events) on decomposition rate of eroded SOM was different depending on the nature of eroding and depositional landform positions considered. The k of organic substrates at 25 cm soil depth in the depositional positions was up to 2 orders of magnitude higher than on the surface of the eroding positions. Results of this litterbag decomposition study suggest that transport of SOM from topsoil of eroding positions to the surface of depositional positions can reduce its k; but burial of eroded SOM in soil profiles at the depositional positions can lead to increasing k. Because erosion-induced C sequestration is a function of changes in rate of OM decomposition and input post-compared to pre-erosion, our findings suggest that higher rates of plant productivity in eroding watersheds is needed to create and maintain a C sink in such eroding watersheds.